In recent years, accompanying the speeding up of information communication, the wiring board is used in a high frequency region of GHz band or more and demanded to be reduced in the transmission loss. To satisfy this requirement, the wiring board is produced by forming a conductor layer composed of a metal having a low conductor resistance and a low melting point, such as silver and copper, on a ceramic substrate having a relatively low dielectric constant. Also, with the progress of high-density packaging or multilayer formation of a circuit, a wiring board having a conductor layer or a via conductor formed by using copper more excellent in the migration resistance than silver is demanded.
In producing a multilayer wiring board, a ceramic layer and a conductor layer are alternately stacked and the conductor layers superposed one on another through a ceramic layer are connected by a via conductor formed to pierce the ceramic layer.
For producing a wiring board by using copper in the conductor layer or via conductor, the organic components must be removed with good efficiency while preventing the oxidation of copper. As the method for realizing this, for example, firing in a wet nitrogen atmosphere (in a mixed atmosphere of water vapor and nitrogen) is known.
More specifically, according to this method, a slurry is prepared using a ceramic raw material powder and an organic binder, a solvent or the like and formed into a ceramic green sheet by a sheet-forming method such as doctor blade method. Thereafter, a via hole is formed on this ceramic green sheet and a copper paste is filled in this via holes and dried to form a via conductor before firing. Furthermore, a conductor lay r working out to a wiring pattern is printed on the ceramic green sheet surface by using a copper paste and dried to form a ceramic green sheet where the via conductor and the conductor layer are connected. Subsequently, a plurality of ceramic green sheets are stacked to form a stacked body and this stacked body is debindered at a temperature of hundreds of ° C. in a mixed atmosphere of water vapor and nitrogen gas to remove organic components contained in the copper paste and ceramic green sheet and then fired by elevating the temperature to nearly 1,000° C. or more. As a result, the conductor layers stacked through a ceramic layer are connected by the via conductor and a multilayer wiring board is produced.
The thus-produced wiring board readily encounters a problem of protrusion that since the firing temperature and the firing shrinkage timing at the firing step differ between the copper working out to a via conductor and the ceramic layer working out to an insulting layer, the via conductor after the firing protrudes from the wiring board surface. As a technique for improving this problem, a copper metallizing composition and a glass ceramic wiring board using the composition disclosed in JP-A-11-53940 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) are known.
In the copper metallizing composition and the glass ceramic wiring board using the composition described in JP-A-11-53940, the glass ceramic porcelain and the copper metallizing composition are matched in the firing shrinkage behavior to reduce the protrusion of the via conductor from the glass ceramic wiring board surface, where the copper metallizing composition which is used for a via hole comprises from 2 to 20 parts by weight of an SiO2—Al2O3—RO(R: alkaline earth metal)—B2O3 based glass frit having a glass transition point of 700 to 750° C. per 100 parts by weight of the copper powder as the main component and the glass ceramic wiring board is obtained by using this copper metallizing composition and firing it simultaneously with a glass ceramic porcelain at a temperature of 700 to 1,000° C.
In the case of enclosing a circuit component such as semiconductor inside the stacked body, a semiconductor element is disposed on the top surface of the wiring board produced above and connected and subsequently, a cover having formed on the bottom surface thereof a housing hole for housing the semiconductor element is formed to cover the semiconductor element.
In using the thus-produced wiring board as a semiconductor package by enclosing a semiconductor, it is important for maintaining stable electric properties to isolate the semiconductor element from the outside water vapor or gas and ensure airtightness. In order to realize this, the interface between a via conductor and a ceramic layer must be prevented from the generation of clearance.
If clearance is generated at the interface between a via conductor and a ceramic layer of a wiring board, when the top surface of ceramic layer or the top surface of via conductor is plated, the plating solution may permeate inside the wiring board through the clearance to damage the wiring circuit. Also for eliminating this problem, the interface between a via conductor and a ceramic layer must be prevented from the generation of clearance.
As a technique for preventing the generation of clearance between a via conductor and a ceramic layer and suppressing the protrusion of a via conductor, a copper metallizing composition and a glass ceramic wiring board using the composition disclosed in JP-A-11-16418 and a metal-containing paste composition for via and a firing method therefor disclosed in JP-A-6-56545 are known.
In the copper metallizing composition and the glass ceramic wiring board using the composition described in JP-A-11-16418, a copper paste having added thereto a glass frit having a softening point of 700 to 1,000° C. is filled in a via hole and fired together with the glass ceramic porcelain at a temperature of 800 to 1,000° C., whereby the adhesive strength between the ceramic porcelain and the via conductor is enhanced to eliminate clearance and at the same time, the protrusion of via conductor is reduced.
In the metal-containing paste composition for via and the firing method therefor disclosed in JP-A-6-56545, this paste composition and a paste composition comprising an electrically conducting first metal, an oxidizable second metal and an organic medium are filled in a via hole and the ceramic porcelain is fired, whereby the clearance between the ceramic porcelain and the via conductor is eliminated.